S. L. Bendell and P. A. Levine in their U.S. Pat. No. 4,435,730 issued Mar. 6, 1984, entitled "LOW NOISE CCD OUTPUT" and assigned to RCA Corporation describe a way to overcome the flicker noise (or 1/f noise) associated with the use of floating-element electrometers as CCD output signal stages. These floating-element electrometers sense the charge on a floating element in a CCD charge transfer channel converting the charge to an output voltage or current. The potential on the floating element, either a floating gate or a floating diffusion, has to be periodically dc-restored to provide a reference level for the voltages electrostatically induced on the element by a charge packet disposed nearby in the charge transfer channel. Floating-diffusion electrometers are favored over floating-gate electrometers, because the floating diffusion can also serve as the source electrode of the metal-oxide-semiconductor transistor used as a clamp in the dc restoration process. This reduces the capacitance associated with the floating element, to increase electrometer sensitivity in accordance with Coulomb's Law. The low-frequency components of the floating-element electrometer response are undesirably contaminated with flicker noise. Bendell and Levine overcame this problem by using a cross-over network. The cross-over network selects only the higher frequency components of the floating-element electrometer output signal as higher frequency components of the final imager output signal, and the cross-over network selects the lower frequency components of the reset drain current as the lower frequency components of the final imager output signal. The reset drain current is typically sensed using a transresistance amplifier with junction field effect transistor (JFET) in common-source amplifier configuration as input stage. The output voltage of the transresistance amplifier, provided in response to reset drain current applied at its input, tends to have poorer high frequency response than the floating-element electrometer. However, there is relatively little 1/f noise contaminating the low-frequency portion of the transresistance amplifier response.
Alternative ways to overcome 1/f noise, which allow obtaining good high frequency response are known. Correlated double sampling is perhaps the best known of these alternative ways. Other ways rely on synchronously detecting the samples of output signal from the CCD imager at a harmonic (e.g. the first harmonic) of the sampling rate.
L. N. Davy in his U.S. Pat. No. 4,330,753 issued May 18, 1982 and entitled "METHOD AND APPARATUS FOR RECOVERING SIGNAL FROM A CHARGE TRANSFER DEVICE" describes a method for obtaining what he characterizes as relatively noise-free information signals from the output stage of a charge transfer device. In the method Davy describes, the output signal from the regularly sampling electrometer stage is passed through a band-pass filter to separate double-sideband amplitude-modulation (DSB AM) sidebands flanking a harmonic of the clocking frequency of the electrometer stage. The separated sidebands are then synchronously detected using a switching demodulator operated at the harmonic of that clocking frequency. The amplitude-modulating signal is heterodyned to baseband spectrum by the switching demodulator. The baseband spectrum of the synchronously detected AM sidebands is separated from the harmonic spectra associated with it and is used as the output signal from the charge transfer device, rather than the baseband spectrum of the imager output signal, which is supressed by the band-pass filtering before synchronous detection. The method Davy describes would be effective in suppressing the 1/f noise in a floating-element electrometer stage, since 1/f noise resides principally in the baseband. It is relatively simple as compared with correlated double sampling to reduce the baseband entirely or at least up to the one or two megahertz frequencies where 1/f noise exceeds the thermal noise background.
P. A. Levine in U.S. patent application Ser. No. 590,044 filed Mar. 15, 1984, now abandoned, entitled "CCD FLOATING-ELEMENT OUTPUT STAGES PROVIDING LOW RESET NOISE WITH SINGLE SAMPLING" and assigned to RCA Corporation describes a synchronous detection of CCD imager output samples after high-pass filtering and differentiation. The synchronous detection is carried out by: sampling the filtered or differentiated response with a switch recurrently conductive at CCD imager output signal clock rate and holding the sampled response on a hold capacitor. Appropriate timing of reset pulses on the floating-element electrometer, this application teaches, reduces reset noise. The reset process in a floating-element electrometer, in which process the floating element is recurrently clamped to a fixed potential, exhibits reset noise owing to the variations in the potential left upon the floating element from one reset interval to another. Reset noise is the predominant noise in the upper-video frequencies of the output signals of charge transfer devices such as CCD imagers, typically being about 8 db larger than noise in the metal-insulator-semiconductor field-effect-transistor (MISFET) electrometer stage following a floating diffusion with 0.07 pf capacitance. At lower video frequencies flicker noise or 1/f noise predominates.
P. A. Levine in U.S. Pat. No. 4,556,851 filed Mar. 21, 1985, entitled "REDUCTION OF NOISE IN SIGNAL FROM CHARGE TRANSFER DEVICES" and assigned to RCA Corporation describes synchronous detection of CCD imager output signals using a different form of reset noise suppression. Resetting of the floating-element is to an in-channel potential, rather than to the direct potential applied to a reset drain.
Synchronous detection of these components of CCD imager output signal that are sidebands of output clock rate, in order to suppress 1/f noise, has been found to have practical problems, however. The synchronous detection is undesirably sensitive to minute variations in the phase relationship between the CCD imager output samples and the switching carrier used in the synchronous detection process. This sensitivity is evidenced in a tendency for spurious low frequency components, causing fixed-pattern shading, to appear sometimes in the video signals derived from the synchronous detector output response. This tendency is most apparent when the CCD imager is operated at low light levels.
The CCD imager output samples tend to be superposed on various low-frequency components prior to their filtering and synchronous detection. However, no amount of filtering to reject baseband frequencies eliminates the spurious low-frequency components in the synchronous detector output response, so these spurious low-frequency components are not attributable to baseband components feeding through the synchronous detector.
Similar low-frequency, fixed-pattern shading manifests itself in correlated double sampling of CCD imager output signal, when the imager is operated at low light levels. This fixed-pattern shading also seems to be a function of sampling pulse phasing. In general, systems tend to be subject to this type of fixed-pattern noise if the CCD imager output signal is not low-passed prior to sampling. So, the origins of this fixed-pattern noise appear to be in the heterodyning of the clocking noise in the CCD imager output signal, which is in the harmonic spectrum of output (C) register clocking signal, with the sampling-pulse frequencies.
The use of synchronous detection of the CCD imager output signal components that are harmonics of output clock rate has been done to suppress 1/f noise. Correlated double sampling has been done to suppress 1/f noise, also. The Bendell and Levine cross-over of imager output signals derived from the floating-element electrometer and from reset drain current sensing has been done to suppress 1/f noise, also. So, no reasons for combining the Bendell and Levine technique with synchronous detection (or with correlated double sampling) have been previously perceived, particularly inasmuch as this would introduce a complication additional to that involved with the use of synchronous detection (or correlated double sampling).
The reset drain current sensing is free of the low-frequency fixed-pattern spurious components that sometimes appear in the synchronous detector output response (or the correlated double sampler output response). This is because there is no wide-band sampling of this imager output signal. So it appears now to the inventor Levine that the use of the low-frequency components of sensed reset drain current in place of the low-frequency components of the synchronous detector (or correlated double sampler) output response would be advantageous from the standpoint of eliminating low-frequency fixed-pattern noise problems. Since the synchronous detection and correlated double sampling schemes are relatively free of 1/f noise, as well as is reset drain current sensing, and since the spurious low frequency components extend only into the tens of kilocycles, the cross-over frequency between reset drain current sensing and the other CCD imager output signal can be set lower than was the practice in the Bendell and Levine technique.
Making the other CCD imager output signal, which crosses-over with the sensed reset drain current, to be the synchronous detector output response or correlated double sampler output response, rather than the baseband component of the floating-element electrometer response, is advantageous in that reset noise can be suppressed. The synchronous detection procedure that suppresses reset noise may be carried out as taught in either of the previously mentioned Levine patent applications, for example.